Add composite model support to build command

[xieofxie]

Summary

Adds composite (seq2seq) model detection and fan-out to winml build, mirroring the pattern already implemented in winml export. Previously, running winml build on a composite model like Qwen3 would attempt a single-model build and fail. Now, composite models are automatically detected and each sub-component is built independently into its own subdirectory.

Changes

src/winml/modelkit/commands/build.py

• Added _warn_partial_composite_build() helper that warns when a composite build fails mid-run, listing completed components without deleting them (same safety semantics as export's _warn_partial_composite).
• In the single-model branch, added composite detection via resolve_composite_components with the same error handling as export:
  • ValueError → UsageError (e.g. ambiguous composite needing explicit --task)
  • RuntimeError re-raised (empty registry guard)
  • OSError suppressed (model ref not resolvable as HF config)
  • Unexpected errors → ClickException
• When composite is detected:
  • Rejects --use-cache (cache keys are ambiguous for multi-component builds)
  • Generates a config per component via generate_build_config(task=component_task)
  • Carries over quant/compile from the outer config (already patched by CLI overrides)
  • Runs _run_single_build per component into output_dir/<component_name>/
  • Tracks completed components for partial-failure warnings

tests/unit/commands/test_build.py

• Added TestBuildComposite class with 6 tests:
  • Fan-out runs one _run_single_build per component into correct subdirectories
  • --use-cache rejected for composite models
  • ValueError during composite resolution surfaces as usage error
  • Unexpected errors during detection are surfaced, not masked
  • Partial build failure warns and keeps completed directories
  • Sub-model configs inherit quant/compile from outer config